The present invention relates to a polymer actuator.
The usefulness of robots is attracting attention in various fields including nursing care service, dangerous work, and entertainment. Robots suitable for these uses are required to have articulations (movable parts) similar to those of animals that permit complex movements.
A conventional actuator to drive these movable parts is a magnetic rotary motor. This actuator, however, suffers the disadvantage of being heavy because it is made of metal.
Weight of actuators built into movable parts add to loads. Heavy actuator needs large outputs, and powerful actuators are large and heavy. This is a difficult-to-solve contradictory problem.
Moreover, magnetic rotary motors require speed reducers to control rotating speed and torque. Speed reducers deteriorate with time as gears therein wear out.
Ultrasonic motors producing a high torque at a low rotating speed do not need speed reducers; but they are also heavy (and hence poses the same problem as mentioned above) because they are made of metal.
For this reason, there have recently been developed polymer actuators in which a light flexible polymeric material plays an important role.
They include polymeric piezoelectric elements (which employ polyvinylidene fluoride), conducting polymer actuators (which employ electron conducting polymers), and gel actuators (which employ polymeric gel).
The gel actuator, particularly the one which employs a water-swelling polymeric hydrogel, relies for its action on a polymeric hydrogel which changes in volume in response to temperature, ionic strength, and pH in its environment. The amount of change in volume is 30 to 50% and the change in volume generates a force of 0.2 to 0.4 MPa. This performance is comparable to that of skeletal muscles.
The polymeric hydrogel, however, has some disadvantages. It cannot be heated or cooled rapidly. It needs an electrolytic solution to control ion concentrations, which has to be circulated by a pump and stored in a reservoir. Consequently, it is not suitable for small, light systems.
There is another type of polymeric hydrogel, which is called a pH-responsive polymeric hydrogel. This hydrogel changes in volume (by electrochemical reactions) depending on the pH of its surrounding aqueous solution.
It is possible to drive and control the pH-responsive hydrogel by utilizing the phenomenon that pH changes in the neighborhood of electrodes and concentration gradient occurs due to electrical double layers formed on electrode surfaces when a voltage is applied across electrodes placed in an electrolytic aqueous solution.
The above-mentioned pH-responsive hydrogel has acidic or basic functional groups in the polymer constituting the gel, so that the gel swells (or changes in volume) depending on pH in the aqueous solution surrounding it.
For example, the one having acidic functional groups works in the following way. When it is in an electrolytic aqueous solution with a high pH, the acidic groups in the gel dissociate protons to become anions, thereby increasing in hydrophilicity and generating repulsive forces in or between negatively charged molecules. This causes the gel to swell. Conversely, in an electrolytic aqueous solution with a low pH, the acidic groups in the gel do not dissociate but form hydrogen bond between them. This causes the gel to shrink.
By contrast, a pH-responsive polymeric hydrogel which has basic groups works in an opposite way. That is, in an electrolytic aqueous solution with a high pH, the basic groups in the gel protonize to become cations, thereby increasing in hydrophilicity and generating repulsive force in or between positively charged molecules. This causes the gel to swell.
Thus, when in use, the pH-responsive polymeric hydrogel is immersed in an electrolytic aqueous solution, and a voltage of about 1 to 3 V is applied across electrodes placed therein. This voltage forms an ion concentration gradient in the electrolytic aqueous solution and changes the pH value in the neighborhood of the electrodes. This mechanism makes it possible to control the swelling and shrinkage of the pH-responsive polymeric hydrogel only with a low voltage without requiring heating and cooling units, pumps, and reservoirs.
The deformation that takes place as mentioned above may be used for an actuator. In fact, there is known an actuator which electrochemically produces curved displacement from a pH-responsive polymeric hydrogel film held between electrodes connected to a voltage source. See Patent Document 1 (JP-B No. Hei-7-97912).
Unfortunately, the actuator of curved displacement type is hardly applicable to robot articulations unlike the actuator capable of linearly expanding and contracting like skeletal muscles. Thus there has been a demand for the actuator which linearly expands and contracts like skeletal muscles.
The gel can be made to expand (elongate) and contract without curving if the distance between electrodes is increased and the gel is brought nearer to one electrode so that the gel is less affected by the other electrode. However, it is very difficult to fix the gel near one electrode while allowing the gel to expand and contract freely.
Application of a voltage across electrodes changes pH near the electrodes. Unfortunately, voltage application brings about the electrolysis of water, which decreases the OH− concentration due to oxidation near the anode, thereby lowering the pH and generating oxygen gas, and which also decreases the H+ concentration due to reduction near the cathode, thereby raising the pH and generating hydrogen gas. The pH change may occur at a place comparatively far from the electrodes when current flowing across the electrodes increases; however, the evolution of oxygen and hydrogen gases and the consumption of water (as a solvent for electrolyte) still pose a problem with designing a closed actuator element.